FORTRAN Logical Expressions

A logical expression can be formed by comparing arithmetic expressions using the following relational operators:

.LT. means less than (<)
.LE. less than or equal (<=)
.GT. greater than (>)
.GE. greater than or equal (>=)
.EQ. equal (=)
.NE. not equal (/=)

So you cannot use symbols like < or = for comparisons in Fortran 77.
For example: (x.eq.y) is valid while (x=y) is not valid in Fortran 77.
Logical expressions can be combined by the logical operators .AND. .OR. .NOT. which have the obvious meaning.

Logical variables and assignment

Truth values can be stored in logical variables. The assignment is analogous to the arithmetic assignment.
Example:

logical a, b
a = .TRUE.
b = a .AND. 3 .LT. 5/2

The order of precedence is important, as the last example shows. The rule is that arithmetic expressions are evaluated first, then relational operators, and finally logical operators. Hence b will be assigned .FALSE. in the example above.
Logical variables are seldom used in Fortran. But logical expressions are frequently used in conditional statements like the if statement.